As an actuator for generating displacement or vibration, a piezoelectric actuator for generating displacement by applying a control voltage to a displacement-generation element using piezoelectric material has been known. A magnetostrictive actuator for generating displacement by applying a magnetic field to a displacement-generation element (referred to as a “magnetostrictive element” hereinafter) using magnetostrictive material has been also known.
A magnetic circuit capable of controlling magnetic field easily by supplying control current thereto is used in the magnetostrictive actuator. Applying the magnetic field generated by the magnetic circuit to the magnetostrictive element allows displacement of the magnetostrictive element to be controlled. As the magnetostrictive material, an Ni magnetostrictive alloy, a Fe—Al magnetostrictive alloy, or a ferrite magnetostrictive alloy have been often used. Further, a rare earth metal and transition metal magnetostrictive supperalloy that generates displacement with an order of magnitude large as compared by these magnetostrictive materials has been developed for commercial use.
The magnetostrictive element generates displacement along its extension or compressive direction based on the strength of the applied magnetic field, not a direction of the magnetic field. Thus, to the magnetostrictive element that generates displacement along its extension direction, for example, an initial displacement based on a static magnetic bias field previously applied can be extended only by an order applied by the magnetic bias field. In a condition where an initial static magnetic bias field is previously applied thereto, if an additional magnetic field generated by the magnetic circuit is applied to the magnetostrictive element, the element can be further displaced along its extension or compressive direction from the initial displacement condition based on the magnetic field generated by the magnetic circuit. In other words, the magnetostrictive element can generate displacement based on a control current. Alternatively, if the magnetostrictive element generates displacement along its compressive direction, the static magnetic bias field is previously applied to the magnetostrictive element so that any initial displacement can be applied to the magnetostrictive element by compressing it by only an order applied by the magnetic bias field, thereby allowing additional displacement to be generated in the magnetostrictive element based on a control current. By disposition of any permanent magnet in a magnet circuit, application of the static magnetic bias field to the magnetostrictive element can be carried out. Namely, the disposition of a permanent magnet in the magnet circuit causes magnetic field generated by the permanent magnet to be applied to the magnetostrictive element as the static magnetic bias field.
If external stress (hereinafter referred to as “prestress”) is previously applied to the magnetostrictive element, larger displacement occurs when applying a magnetic field thereto as compared by a case where no prestress is applied thereto. Thus, any prestress has been often applied to magnetostrictive actuator using any magnetostrictive element to increase its displacement. The prestress depends on material constituting the magnetostrictive element. For example, in a Ni magnetostrictive alloy, displacement occurs in its compressive direction so that an initial strain stress is applied as prestress, thereby obtaining larger displacement. In each of the Fe—Al magnetostrictive alloy, ferrite magnetostrictive alloy, and rare earth metal and transition metal magnetostrictive supperalloy, on the other hand, displacement occurs in its extension direction so that an initial compressive stress is applied as prestress, thereby obtaining larger displacement.
Japanese Patent Application Publication No. H07-15053 discloses a magnetostrictive actuator, as shown in FIG. 1, including a rod-like magnetostrictive element 31 made of magnetostrictive alloy, a solenoid coil 32, which is arranged around the magnetostrictive element 31, for applying controlling magnetic field to the magnetostrictive element 31, a driving rod 33 connected to an end of the magnetostrictive element 31 to propagate any displacement outputs of the magnetostrictive actuator, a fixed base plate 34 connected to the other end of the magnetostrictive element 31, a tubular case 35 attached to the fixed base plate 34 so as to be arranged around the solenoid coil 32. A permanent magnet 36 is attached to the fixed base plate 34. The permanent magnet 36 allows the magnetic bias field to be applied to the magnetostrictive element 31.
The driving rod 33 and the cylinder-like case 35 are connected to each other thorough the flexible connecting part 37. The flexible connecting part 37 is connected to the driving rod 33 and the tubular case 35 so that it can be flexible in a driving direction of the magnetostrictive actuator but solid in a direction perpendicular to the driving direction. This connecting part 37 has a configuration such that it has little effect on the displacement output of the magnetostrictive element 31, thereby realizing a design to decrease in a loss of the displacement output thereof.